Solid-liquid partitioning, advantages

Liquid-Liquid Partitioning of Fine Solids This process involves separation of small-particle solids suspended in a feed hquid, by contact with a second liquid phase. Robbins describes such a process for removing ash from pulverized coal [U.S. Patent 4,575,418 (1986)]. The process involves slurrying pulverized coal fines into a hydrocarbon liquid and contacting the resulting slurry with water. The coal slurry is cleaned by preferential transfer of ash particles into the aqueous phase. The process takes advantage of differences in surfacewetting properties to separate the different types of solid particles present in the feed. 

A saturation column situated between the pump and the injection device may be installed for two reasons. In a liquid-liquid partition system a saturation column containing a large amount of stationary phase on a suitable solid support may be used in order to ensure a proper equilibrium between the two phases. In other systems a saturation column containing bare silica may be installed in order to prevent dissolution of silica from the analytical column. This is an advantage even if the analytical colunm contains chemically modified silica for reversed phase chromatography. 

Normal phase liquid partition. The polar characteristics of the stationary phase can be modified by incorporating ether, nitrile, nitro, diol and/or amino substituents normally at the end of a hydrocarbon chain or, on an aromatic ring, both of which are chemically bonded to the support material. The major advantage of these materials compared to the liquid-solid systems described above is the facility to undertake gradient elution. Elution is normally carried out with relatively non-polar solvents such as tetrahydrofuran, diethyl ether, chloroform and hexane. 

Both gas/solid adsorption and gas/liquid partition chromatography can be used for GC-MS, but GC is by far the most common. Because, in GC, the stationary phase is a liquid, usually a polymer, its vapor pressure will cause a continual low flow, or bleed into the ion source of the mass spectrometer. This bleed, which usually consists of decomposed stationary phase, will produce a spectrum whose intensity increases with column temperature. Stationary phases should therefore be of the high-boiling, low-bleed type. Most currently used stationary phases for routine GC-MS are based on alkyl-polysiloxanes or alkyl-phenyl-polysiloxanes that are chemically bonded to the column wall to increase stability. Columns containing such phases can, in some cases, be used at temperatures of up to 400°C. One advantage, however, to the presence of bleed peaks in the spectrum is that they enable a continual check to be made on the mass spectrometer calibration. For the alkyl siloxanes, ion peaks are present, in decreasing relative abundance, at miz 73, 207, 281, 355, 429,... 

Various countercurrent chromatographic techniques have been successfully employed for the separation of flavonoids. Countercurrent chromatography is a separation technique that relies on the partition of a sample between two immiscible solvents, the relative proportions of solute passing into each of the two phases determined by the partition coefficients of the components of the solute. It is an all-liquid method that is characterized by the absence of a solid support, and thus has the following advantages over other chromatographic techniques ... 

In certain cases, the separation medium of a membrane is a liquid that is immiscible with the feed stream. The very high permeability of liquids relative to solid materials offers a productivity advantage. Usually the selectivity of liquids derives from differential partitioning of permeants. Liquids may also be used as a solvent for specific com-plexing agents that do not form membranes themselves. Finally, transient deposits of colloids can be used as selective barriers in the so-called dynamic membranes, which offer very high productivities when moderate degrees of separation are adequate. 

High-performance liquid chromatography (HPLC) is a separation technique based on a solid stationary phase and a liquid mobile phase. Separations are achieved by partition, adsorption, exclusion, or ion-exchange processes, depending on the type of stationary phase used. HPLC has distinct advantages over gas chromatography for the analysis of nonvolatile organic compounds. Compounds to be analyzed are dissolved in a liquid, and most separations take place at room temperature. 

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